The invention relates to a method of improving the quality of soldered connections between large-area SMD components with contact pads on the underside and circuit boards or wiring carriers, in the case of which the solder required for connection is already provided on at least one of these parts to be joined, a flux is used to activate the solder, and the electrical and mechanical connection is established by a soldered connection by means of heat exposure and melting of the solder-flux mixture while the parts to be joined are transported through a soldering furnace, with a subsequent cooling phase.
Large-area SMD components, such as area-array components, are for example unpackaged semiconductor components (flip-chip components) or BGA (Ball-Grid-Array), BOC (Board-on-Chip) and CSP (Chip-Size-Packaging) components packaged in the smallest space by using wiring techniques, in the case of which the electrical terminals (pads) present on the underside are electrically connected to contacts on a printed circuit board with the aid of a soldered connection. To establish these soldered connections, usually solder is firstly applied to at least one of the parts to be joined and the component is subsequently positioned at the intended mounting location. The soldered connection is established after that by reflow soldering in a soldering furnace, a flux being required to activate the solder. This flux, usually comprising organic constituents, is either applied directly to one of the two parts to be joined, or is applied in the form of a solder paste as a mixture of small solder particles, usually with further organic additives (solvents and binders), to the location to be connected, for example by printing or dispensing. Printing of the solder paste allows either part or the entire amount of the solder required for establishing the connection, including the required flux, to be provided.
However, it must be stated that, when melting the solder paste in a customary reflow process, not all the organic constituents contained in the solder paste are used up for the chemical activation of the parts to be joined and the solder particles. The flux, solder and binder that remain in this case are distributed in and around the soldered connection during the soldering operation. The organic constituents still present within the soldered connection which is produced form bubbles filled with liquid or vapour during the soldering operation and, on account of their buoyancy, rise up into the upper regions of the soldered connection, since they are significantly lighter than the liquid metal surrounding them.
This has the consequence that, depending on the position of the subassembly, the bubbles impinge on the usually flat component terminals (pads), the surfaces of the underside of the component or the surface of the printed circuit board as it runs through the soldering furnace. By contrast with open soldered connections, the bubbles cannot escape from the soldered connection in their natural direction of movement. A lateral drift out of the soldered location is only possible to a greatly retarded extent, if at all. It has been found that many bubbles become lodged in the region under the soldered terminals and are entrapped in the solidified solder at the end of the soldering phase. The defects produced in the soldered contact in this way are known as “voids”.
These voids disturb the electrical and mechanical function of the soldered connection if their number or size becomes too great in comparison with the amount of solder. A further disadvantage of these voids is that the organic constituents remaining in them have the effect over time of making the soldered connection come apart during the operation of the component as a result of thermal expansion. Harmful substances escaping or gathering in voids may lead to slow destruction of the subassembly caused by corrosion and lead to electrical short circuits caused by ions.
It has been found that a particularly large number of voids are formed under unfavourable conditions if, in addition to the flat component terminals, the soldered joint is also covered by the rest of the component. This is the case in particular with the area-array components used to an increasing extent, such as flip-chip, BGA, CoB or CSP components, in the case of which there are a particularly high number of soldered joints on the underside, distributed over the surface area. Here, the amount or concentration of the remains of the flux, solvent and binder contents of the soldered connection is particularly high in comparison with the amount of metal if too much solder or the entire solder required for the connection has been provided in the form of solder paste (mixture of approximately 50% by volume of organic constituents and 50% by volume of metal).
A particularly high number of voids are also formed if, on account of the further development of subassemblies, relatively small soldered connections have to be realized, but the size of the bubbles remains the same on the basis of the physical properties. In this case, the number and size of the voids in the soldered connections may reach critical ranges and can no longer be ignored, since they can then lead to reliability being restricted or render them completely useless.
To avoid such restrictions of the reliability and function of electrical components caused by voids, various tests have been carried out to research the causes of the voids and, finally, produce soldered connections that are free from voids.
In this respect, the following publications have become known:
W. Casey, Reduction of BGA Eutectic Ball Solder Joint Voiding, Proc. SMI '98 San Jose, 23-27 Aug. 1998; H. Bell, M. Kämpfert, Haken und Ösen bei der Verarbeitung von BGAs, Teil 1 und 2 [snags in the processing of BGAs, parts 1 and 2], SMT 2001 issue 3, pages 30-33 and issue 4, pages 12-17; N. C. Lee, Troubleshooting the Reflow soldering for SMT, BGA and Flip Chip Processes, Nepcon West 1998, Anaheim USA Calif.; M. Reichenberger, D. Kozic, H. Roth, Bleifrei geht's auch, Fehlstellen vermeiden in bleifreien BGA-Lötstellen [lead-free works too; avoid defects in lead-free BGA soldered joints], F & M Feinwerktechnik Microtechnik Microelektronik 19 (2001) 4.
Specified in these publications are various causes for the production of voids and their avoidance. For instance, changing the composition of the solder pastes by lowering the flux and solder content or increasing the metal content, or changing the viscosity of the organic constituents and the chemical composition of the solder pastes and also the grain size of the solder balls, is described. This achieves the effect that the volume and number of voids are reduced and the buoyancy or migration velocity is increased. As a result, however, there is a deterioration in the processing properties of the solder paste, such as the behaviour in screen printing and the wetting characteristics.
Another possibility is to change the nucleation for bubbles on the surfaces of the parts to be joined due to impurities, condensation water, moisture or surfaces with reactive regions or roughnesses. However, this requires additional cleaning operations, the avoidance of temperature fluctuations or the reduction of atmospheric humidity, and consequently leads to considerable additional effort.
The design of the parts to be joined may also be changed, by reducing the form and location of the ring of solder stop lacquer around the joint, in order to avoid a build-up of the bubbles escaping. Although this sometimes allows quite good results to be achieved, it does not produce an optimum design with respect to other properties of the subassembly.
Changing the melting profile has also been attempted, in order to give the bubbles the time to rise up and escape from the metal melt to the sides by raising the temperatures more slowly or more quickly and changing the peak temperature. However, the significantly increased thermal loading of the subassembly is disadvantageous in this case.
Finally, it has also been attempted to change the atmospheric conditions, by carrying out the soldering operation in a nitrogen atmosphere, in order to reduce the surface tension of the solder and consequently the work of emission. However, the formation of voids cannot be completely avoided in this way.
The main disadvantages of these methods are the increased effort, the deterioration in the quality of the subassembly and the inadequate avoidance of the occurrence of voids.
The invention is therefore based on the object of providing a method of improving the quality of soldered connections which is simple to realize and particularly effective and with which the number of voids can be drastically reduced.